Biaxially oriented polypropylene (BOPP) films used for packaging, decorative, and label applications often perform multiple functions. For example, in laminations they can provide printability, transparent or matte appearance, and/or slip properties. They can further be used to provide a surface suitable for receiving organic or inorganic coatings for gas and moisture barrier properties. They can also be used to provide a heat sealable layer for bag forming and sealing, or a layer that is suitable for receiving an adhesive either by coating or laminating.
However, in recent years, interest in “greener” packaging has been developing. Packaging materials based on biologically derived polymers are increasing due to concerns with renewable resources, raw materials, and greenhouse gases. Bio-based polymers are believed—once fully scaled-up—to help reduce reliance on petroleum, reduce production of greenhouse gases, and can be biodegradable. The biodegradable aspect is of interest to many snack manufacturers so as to provide litter abatement in addition to a lower carbon footprint package. Bio-based polymers such as polylactic acid (PLA)—which is currently derived from corn starch (but can be derived from other plant sugars) and thus, can be considered to be derived from a renewable or sustainable resource—is one of the more popular and commercially available materials available for packaging film applications. Other bio-based polymers such as polyhydroxyalkanoates (PHA) and particularly, polyhydroxybutyrate (PHB) are also of high interest.
For such a bio-based polymer to be fit-for-use for many snack food packaging applications, it is desirable that the bio-based polymer film match as many of the attributes possible that BOPP is well-known for, such as heat sealability, printability, controlled COF, metallizability, barrier, etc. In particular, for high barrier packaging, metallized oriented PLA films should demonstrate good oxygen and moisture barrier properties. For metallized oriented PLA in particular, good oxygen barrier property is generally easily achieved due to the polar nature of PLA, which provides good hydrogen-bonding of the polymer molecules. However, this polar nature tends to be detrimental for achieving high moisture barrier. Without being bound by any theory, the thought is that water molecules—being polar themselves—may more easily migrate through a polar polymer film than a non-polar polymer film.
There are several manufacturers of oriented PLA films, in particular biaxially oriented PLA, but none of them provides a satisfactory moisture barrier property when metallized. For example, the data sheet for Celplast Metallized Products, Ltd.'s Enviromet™ high barrier metallized PLA film describes a product that exhibits an excellent oxygen barrier of 6.2 cc/m2/day (at 23° C., 50% relative humidity or RH) but a relatively poor moisture barrier of 3.1 g/m2/day (at 38° C., 90% RH) as compared to typical metallized biaxially oriented polypropylene films. (High barrier metallized BOPP such as Toray Plastics (America), Inc.'s PWX3 product typically demonstrates oxygen barrier of 15.5 cc/m2/day (23° C., 0% RH) and moisture barrier of 0.155 g/m2/day (38° C., 90% RH).)
Another manufacturer of barrier PLA film, Alcan Packaging Inc., produces a silicon oxide coated PLA film under the tradename Ceramis®. The data sheet for Ceramis® states that this product has an oxygen barrier of 7.75 cc/m2/day (23° C., 50% RH) and moisture barrier of 7.75 g/m2/day (38° C., 90% RH). Biofilm S.A. promotional literature (such as presented at the “Innovation Takes Root” conference hosted by NatureWorks LLC at Las Vegas, Nev. Sep. 16-18, 2008) discusses transparent barrier PLA films demonstrating 3-10 g/m2/day (38° C./90% RH) using various vacuum chamber deposition processes. For both the latter transparent barrier PLA films, vacuum deposition of a metal such, as aluminum on top of the transparent barrier layer (e.g. silicon oxide, aluminum oxide, or other coatings), may be used to improve moisture barrier properties, but such a product is likely to be costly due to the multiple processing steps required.
U.S. application Ser. No. 10/400,056 describes a multi-layer metallized film including a polyolefin core layer (typically of polypropylene), a polyolefin-based tie-layer on one side of the core layer, and a metal receiving layer of PLA on the tie-layer opposite the core layer side. The PLA layer is metallized. Gas barrier properties of this metallized film are very good, with the examples citing moisture barrier as 0.054-0.093 g/m2/day (38° C., 90% RH) and oxygen barrier as 1.09-1.24 cc/m2/day (23° C., 0% RH). However, such a film formulation is primarily based on petrochemicals (such as polypropylene or copolymers thereof), being 96-98 wt % of the total film structure. PLA bio-polymer is only 2-4 wt % of the film. Thus, such a film cannot claim to be a “sustainable” film, being made from a majority amount of petroleum-based resins versus bio-based resins, and also will not be biodegradable.
EP Patent Publication No. 02065178/EP-A1 describes substrates of PET, PEN, and PLA films over-coated with a polyurethane-based coating which is then subjected to a vapor-deposition process in which a metal anchor layer is deposited upon the polyurethane coating and then an inorganic coating deposited upon the metal anchor layer. Gas barrier properties are improved by using this particular type of polyurethane coating with the inorganic layer. However, the improvement of gas barrier properties on oriented PLA substrates without the polyurethane coating is not contemplated.
U.S. Pat. No. 5,147,726 describes capacitor films of PET, polypropylene, polyethylene naphthalate, polyetheretherketone, and polyphenylenesulfide made by a process wherein a primer layer of copper can be vacuum deposited on the substrate prior to metallization with aluminum. This technique improves the strength of the deposited layer. However, this patent recommends the process as being most effective on polyethylene terephthalate (PET) films and does not contemplate PLA films nor the improvement of gas barrier properties on PLA films.
U.S. application Ser. No. 10/536,329 describes transparent aluminum oxide-coated substrates such as PET which are provided with an “ultrathin” dusted metal or metal oxide layer prior to the vapor-deposition of aluminum oxide. The term “ultrathin” means a thickness of a layer that does not allow the formation of a closed layer. Barrier properties of transparent aluminum oxide coated substrates can be improved by this method. However, the application does not contemplate using biopolymers such as PLA or non-transparent, opaque inorganic coatings.
U.S. application Ser. No. 12/332,153 describes biaxially oriented PLA film designs combining polyolefin metal receiving layers with polylactic acid polymer core layer for improved moisture barrier properties after metallizing. However, such a structure is not composed entirely of biodegradable or compostable polymers.
U.S. application Ser. No. 12/333,047 describes a method to produce biaxially oriented PLA films at high transverse orientation rates using a specific processing aid. This reference is incorporated in its entirety in this application.